2009 was tied for the second warmest year in the modern record, a new
NASA analysis of global surface temperature shows. The analysis,
conducted by the Goddard Institute for Space Studies (GISS) in New York
City, also shows that in the Southern Hemisphere, 2009 was the warmest
year since modern records began in 1880.

Although 2008 was the coolest year of the decade — due to strong
cooling of the tropical Pacific Ocean — 2009 saw a return to
near-record global temperatures. The past year was only a fraction of a
degree cooler than 2005, the warmest year on record, and tied with a
cluster of other years — 1998, 2002, 2003, 2006 and 2007 — as the
second warmest year since recordkeeping began.

“There’s always an interest in the annual temperature numbers
and on a given year’s ranking, but usually that misses the
point,” said James Hansen, the director of GISS. “There’s
substantial year-to-year variability of global temperature caused by the
tropical El Niño-La Niña cycle. But when we average temperature
over five or ten years to minimize that variability, we find that global
warming is continuing unabated.”

January 2000 to December 2009 was the warmest decade on record.
Throughout the last three decades, the GISS surface temperature record
shows an upward trend of about 0.2°C (0.36°F) per decade. Since
1880, the year that modern scientific instrumentation became available
to monitor temperatures precisely, a clear warming trend is present,
though there was a leveling off between the 1940s and 1970s.

The near-record temperatures of 2009 occurred despite an unseasonably
cool December in much of North America. High air pressures in the Arctic
decreased the east-west flow of the jet stream, while also increasing
its tendency to blow from north to south and draw cold air southward
from the Arctic. This resulted in an unusual effect that caused frigid
air from the Arctic to rush into North America and warmer mid-latitude
air to shift toward the north.

“Of course, the contiguous 48 states cover only 1.5 percent of the world
area, so the U.S. temperature does not affect the global temperature
much,’ said Hansen.

In total, average global temperatures have increased by about 0.8°C
(1.4°F) since 1880.

“That’s the important number to keep in mind,” said Gavin
Schmidt, another GISS climatologist. “In contrast, the difference
between, say, the second and sixth warmest years is trivial since the
known uncertainty — or noise — in the temperature measurement is
larger than some of the differences between the warmest years.”

Decoding the Temperature Record

Climate scientists agree that rising levels of carbon dioxide and other
greenhouse gases trap incoming heat near the surface of the Earth and
are the key factors causing the rise in temperatures since 1880, but
these gases are not the only factors that can impact global
temperatures.

Three others key factors — including changes in the sun’s
irradiance, oscillations of sea surface temperature in the tropics, and
changes in aerosol levels — can also cause slight increases or
decreases in the planet’s temperature. Overall, the evidence suggests
that these effects are not enough to account for the global warming
observed since 1880.

El Niño and La Niña are prime examples of how the oceans can
affect global temperatures. They describe abnormally warm or cool sea
surface temperatures in the South Pacific that are caused by changing
ocean currents.

Global temperatures tend to decrease in the wake of La Niña, which
occurs when upwelling cold water off the coast of Peru spreads westward
in the equatorial Pacific Ocean. La Niña, which moderates the impact
of greenhouse-gas driven warming, lingered during the early months of
2009 and gave way to the beginning of an El Niño phase in October
that’s expected to continue in 2010.

An especially powerful El Niño cycle in 1998 is thought to have
contributed to the unusually high temperatures that year, and
Hansen’s group estimates that there’s a good chance 2010 will be
the warmest year on record if the current El Niño persists. At most,
scientists estimate that El Niño and La Niña can cause global
temperatures to deviate by about 0.2°C (0.36°F).

Warmer surface temperatures also tend to occur during particularly
active parts of the solar cycle, known as solar maximums, while slightly
cooler temperatures occur during lulls in activity, called minimums.

A deep solar minimum has made sunspots a rarity in the last few years.
Such lulls in solar activity, which can cause the total amount of energy
given off by the sun to decrease by about a tenth of a percent,
typically spur surface temperature to dip slightly. Overall, solar
minimums and maximums are thought to produce no more than 0.1°C
(0.18°F) of cooling or warming.

“In 2009, it was clear that even the deepest solar minimum in the
period of satellite data hasn’t stopped global warming from
continuing,” said Hansen.

Small particles in the atmosphere called aerosols can also affect the
climate. Volcanoes are powerful sources of sulfate aerosols that
counteract global warming by reflecting incoming solar radiation back
into space. In the past, large eruptions at Mount Pinatubo in the
Philippines and El Chichón in Mexico have caused global dips in
surface temperature of as much as 0.3°C (0.54°F). But volcanic
eruptions in 2009 have not had a significant impact.

Meanwhile, other types of aerosols, often produced by burning fossil
fuels, can change surface temperatures by either reflecting or absorbing
incoming sunlight. Hansen’s group estimates that aerosols probably
counteract about half of the warming produced by man-made greenhouse
gases, but he cautions that better measurements of these elusive
particles are needed.

Data Details

To conduct its analysis, GISS uses publicly available data from three
sources: weather data from more than a thousand meteorological stations
around the world; satellite observations of sea surface temperature; and
Antarctic research station measurements. These three data sets are
loaded into a computer program, which is available for public download
from the GISS website. The program calculates trends in temperature
anomalies — not absolute temperatures — but changes relative to the
average temperature for the same month during the period of 1951-1980.

Other research groups also track global temperature trends but use
different analysis techniques. The Met Office Hadley Centre, based in
the United Kingdom, uses similar input measurements as GISS, for
example, but it omits large areas of the Arctic and Antarctic, where
monitoring stations are sparse.

In contrast, the GISS analysis extrapolates data in those regions using
information from the nearest available monitoring stations, and thus has
more complete coverage of the polar areas. If GISS didn’t extrapolate in
this manner, the software that performs the analysis would assume that
areas without monitoring stations warm at the same rate as the global
mean, an assumption that doesn’t line up with changes that satellites
have observed in Arctic sea ice, Schmidt explained. Although the two
methods produce slightly different results in the annual rankings, the
decade-long trends in the two records are essentially identical.

“There’s a contradiction between the results shown here and popular
perceptions about climate trends,” Hansen said. “In the last decade,
global warming has not stopped.”